A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 541 



green, strongly pleochroic variety, and is disposed about the augite in such a way as 

 to indicate that it is due to molecular instability of the pyroxene during or after 

 crystallisation. The hornblende of the Ochilesa rock (1766) is of a reddish-brown 

 tint. Some of the hypersthene shows a close lamellar twinning and a small amount 

 of schillerisation. The rounded grains of iron-ore are embedded in the pyroxenes, 

 and in the augite they form centres from which amphibolisation has started. These 

 rocks are hornblende-hyperites approaching melanocratism, in which the amount of 

 plagioclase and quartz is estimated to be only half that of the ferro-magnesian 

 minerals (see Table I, 3, 4). They belong to Holland's basic division of the char- 

 nockite series. A similar rock (192) occurs near Andulo, and is distinguishable only 

 by its finer grain. 



Holland's ultrabasic division of the charnockite series is not represented in this 

 collection, but a thoroughly melanocratic hornblende-hyperite from Ochilesa (176a) 

 forms an approach to that division. The proportion of ferro-magnesian minerals to 

 plagioclase in this rock is estimated at three to one (see Table I, 5) ; otherwise the 

 rock differs little from the hyperite described above. 



This completes the description of the series comparable with the charnockites of 

 India. One other rock undoubtedly belongs to this series, but has no Indian equiva- 

 lent. This rock is from the W. of Chieuca (164), and is interstratified with well- 

 foliated gneiss. It may be described as a charnockite-porphyry. It consists of small 

 phenocrysts of quartz, oligoclase, orthoclase, augite, hypersthene, and biotite, pro- 

 fusely scattered over a minute, evenly granular, microcrystalline groundmass com- 

 posed mainly of quartz grains with subordinate water-clear felspar. The augite is 

 well schillerised, and has fringes of greenish-brown hornblende. Biotite occurs in 

 large clots consisting of small flakes felted with granules of augite and magnetite. 

 It has exactly the appearance of the new-formed flakes in a hornfelsed rock, and the 

 whole aspect of the rock suggests that it is secondary. Oligoclase forms the largest 

 and most abundant set of phenocrysts. The rock appears to be the hypabyssal 

 equivalent of the Ochilesa charnockite. The granulitic texture, the extreme fresh- 

 ness, and the presence of abundant new-formed biotite, indicate that this rock has 

 suffered some thermal metamorphism. It is linked by these characters, as well as 

 others, to the hornfelsed porphyries described in the next paragraph. 



The quantitative mineralogical composition or mode of this series has been 

 obtained by the Rosiwal method of measurement on five of the rock types, with 

 the results shown in Table I. 



[Table 



